Support repo code search without setting up an indexer (#29998)

By using git's ability, end users (especially small instance users) do
not need to enable the indexer, they could also benefit from the code
searching feature.

Fix #29996


![image](https://github.com/go-gitea/gitea/assets/2114189/11b7e458-88a4-480d-b4d7-72ee59406dd1)


![image](https://github.com/go-gitea/gitea/assets/2114189/0fe777d5-c95c-4288-a818-0427680805b6)

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Co-authored-by: silverwind <me@silverwind.io>

options/locale/locale_en-US.ini

@@ -172,6 +172,7 @@ org_kind = Search orgs...
 team_kind = Search teams...
 code_kind = Search code...
 code_search_unavailable = Code search is currently not available. Please contact the site administrator.
+code_search_by_git_grep = Current code search results are provided by "git grep". There might be better results if site administrator enables Repository Indexer.
 package_kind = Search packages...
 project_kind = Search projects...
 branch_kind = Search branches...